For improving a dezincification resistance among corrosion resistances of α+β brass and α+β+γ brass, it is conventionally general that crystallization of a β phase which is electrically lowest in the metallographic structure is suppressed by lowering the Zn equivalent to relatively increase the Cu content, and the remaining β phase is allowed to undergo α transition by a thermal treatment. Additionally, it is known that it is effective to add P, Sb, As and Sn as an additive element.
For improving a stress corrosion cracking resistance among corrosion resistances, addition of Pb which is electrically lower and not solid-solved in a matrix is most general, however, in the case of a lead-free copper alloy, it is known that it is effective to add 1.0% or more of Sn as an alternative to Pb because the Pb content should be suppressed. Addition of Sn, however, leads to lowering of cold mechanical properties such as elongation and impact value, thus, problems occur in cold working and cutting work in some cases. Then, in alloy design, the Sn content is suppressed to the minimum necessary, in view of practical ranges of elongation and impact value.
As described above, very delicate alloy design is required for controlling corrosion resistance and characteristics such as mechanical properties in practically effective ranges in a lead-free copper alloy. Also in the past, a corrosion resistance of a forged product is studied in view of these points.
For improving corrosion resistances of a forged product by a thermal treatment, for example, a copper alloy containing 60 to 63% by mass of Cu is subjected to a thermal treatment at 350° C. to 650° C. after hot forging, thereby causing α transition of the structure, trying to improve a dezincification resistance, in Patent document 1. Further, the same document describes that when gradual cooling at a rate of 10° C./sec or less is conducted after hot forging, a β phase is fragmented and surrounded by an α phase, resulting in an improvement in a dezincification resistance.
In Patent document 2, corrosion resistances of a lead-free brass alloy are intended to be improved, and a brass alloy containing 55 to 65% by weight of copper is subjected to extrusion at a temperature of 630 to 720° C., then, to an intermediate thermal treatment at a temperature of 420 to 700° C., then, to annealing at a temperature of 400° C. or lower, to eliminate stress.